In traveling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve the desired interaction, the electromagnetic wave is propagated along a slow-wave structure, such as an electrically conductive helix wound around the path of the electron stream. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure so that the traveling wave may be made to effectively propagate at nearly the velocity of the electron stream. Slow-wave structures of the helix type are usually supported within an encasing barrel by means of a plurality (usually three) of equally circumferentially spaced electrically insulating rods positioned around the helix and within the barrel.
In order to reduce the variation in phase velocity as a function of frequency and thereby increase the operating bandwidth, slow-wave structures have been loaded with longitudinally extending conductors projecting radially inwardly from the encasing barrel. One prior technique for producing such conductive loading is to form the barrel by wrapping a conductive wire about the slow-wave structure and its support rods, thereby producing a triangulation effect in which conductive material is located closer to the slow-wave structure in regions between the rods. Another prior technique, disclosed in U.S. Pat. No. 3,972,005 to Nevins, Jr., et al, involves mounting a plurality of individual radially inwardly projecting conductive vane members between the support rods.
As traveling-wave tube operating frequencies increased, required dimensions for slow-wave structures employed in such tubes have become smaller and smaller. The aforementioned conductive loading methods are extremely difficult to carry out and have not been satisfactory for small-sized slow-wave structures, such as those operating at millimeter wavelengths.